In general, documents US 2004/0075585, EP 0 693 670, DE 44 14 737, and US 2009/0097014 describe for some of them systems for determining the length of optical fibers and for the other ones systems for weapon tracking.
An underwater weapon is for example formed by a torpedo or by any other type of underwater missile or with change of medium fired or launched from an underwater vehicle, such as a submarine strictly speaking.
The free end of the fiber is connected to the submarine and during displacements of the torpedo the fiber is then unwound from the reel of the weapon and is used as a medium for transmitting information between the vehicle and the weapon in order to for example ensure fiber guidance during at least one portion of its displacement for example towards a target.
Generally, it is known to use the length of a wire unwound behind a torpedo in order to know the distance covered by the latter since its launching, and in particular, this has been applied to wire-guided torpedoes which unwind a copper wire behind them.
The coating of this copper wire then has magnetic means at regular intervals which, during the unwinding of the wire, give the possibility of counting beeps upon the passing of these means in front of the corresponding sensors of the torpedo, and therefore of reconstructing the distance covered by the weapon.